1. Field of the Invention
The present invention relates to a sheet supply apparatus for supplying a sheet (recording sheet, transfer sheet, photo-sensitive sheet, electrostatic recording sheet, printing sheet, OHP sheet, envelope, post card, original sheet or the like) from a sheet stacking portion to a sheet treating portion (such as a recording portion, a reading portion, working portion or the like) in a recording apparatus (printer) acting as an information outputting apparatus of a word processor, a personal computer and the like, or in an image forming apparatus such as a copying machine, a facsimile and the like, or other equipments using the sheet, and a recording apparatus having such a sheet supply apparatus.
2. Related Background Art
In sheet supply apparatuses, a function for surely separating a single sheet from a sheet stack is requested. In the past, there has been proposed a technique in which a pawl member is arranged at a front corner of the sheet stack so that, when the sheets are fed out by a sheet supply roller, by flexing only an uppermost sheet to ride over the pawl member, the uppermost sheet is separated from the other sheets. However, even when this technique is used, it is very difficult to separate a sheet which is hard to be flexed (for example, an envelope or a post card having strong resiliency).
On the other hand, in order to separate the sheet which is hard to be flexed (such as an envelope or a post card), a technique is proposed as disclosed in the Japanese Patent Application Laid-open No. 3-284547 (1991). This technique will now be explained with reference to FIG. 28. In FIG. 28, a sheet stacking plate 201 on which sheets are stacked is biased upwardly by a spring member 203. A free roller 204 for regulating a position of an uppermost sheet on the sheet stack is abutted against an upper surface of the sheet stack rested on the sheet stacking plate 210 so that the upper surface of the sheet stack is maintained below a guide surface 205. Further, an inclined surface 207 for separating the sheets is arranged at a downstream side of the sheet stacking plate 201.
A sheet supply roller 206 is a semi-circular roller having a large diameter portion and a small diameter portion. During rotation of the sheet supply roller, when the large diameter portion thereof is contacted with the uppermost sheet on the sheet stack, the sheets are fed out. The sheets fed out by the sheet supply roller 206 are urged against the inclined surface 207, and the uppermost is flexed to ride over the inclined surface 207, thereby separating the uppermost sheet from the other sheets. Since tip ends of the second, third and other sheets are held down by an elastic force of the flexed uppermost sheet, the second, third and other sheets cannot ride over the inclined surface 207. In this way, only the uppermost sheet can surely be separated from the other sheets.
However, in such a sheet separating mechanism, since the tip ends of the second, third and other sheets are held down by the elastic force generated when the sheet is flexed between the inclined surface 207 and a point P (contact point between the sheet and the free roller 204), and, thus, since the elastic force affects a great influence upon the separating operation, it is necessary to select an inclination angle of the inclined surface 207 in accordance with the bending elastic modulus of the sheet. That is to say, when a sheet having the great bending elastic modulus is separated, the inclination angle of the inclined surface must be selected to be smaller so as not to fold the sheet to be fed out; whereas, when a sheet having the small bending elastic modulus is separated, the inclination angle of the inclined surface must be selected to be greater so as to surely hold down the other sheets by the elastic force of the flexed uppermost sheet.
Accordingly, if the inclination angle of the inclined surface 207 is selected to be smaller to permit the separation of the sheet having the great bending elastic modulus (such as an envelope, a post card or the like), for example, when it is desired to separate a sheet (for a copying machine) having a weight of 60-100 grams/m.sup.2, the second, third and other sheets cannot be sufficiently held down by the elastic force of the flexed uppermost sheet, with the result that the double-feed of sheets may occur. Thus, this arrangement cannot be used in separation of the sheet (such as plain sheet) having the small bending elastic modulus.
To avoid this, there has been proposed a technique in which plural kinds of sheets having each different bending elastic modulus can be separated by a single separation means, for example, as disclosed in the Japanese Patent Application Laid-open No. 58-202228 (1983). Now, this technique will be briefly explained with reference to FIG. 29.
A sheet stacking plate 301 on which sheets are stacked is biased upwardly by a spring 302, and a position of an uppermost sheet on the sheet stack is regulated by holder pawls 302 disposed in the proximity of left and right front corners of the sheet stack. A sheet supply roller 303 is urged against the uppermost sheet so that, when the sheet supply roller is rotated, the sheet can be fed out. An abutment member 305 provided on a reference surface 304 for regulating tip ends of the stacked sheets is formed from a plastic film or a metal spring plate having a predetermined bending elastic modulus so that the abutment member can be bent or flexed when it is urged by the sheets fed out by the sheet supply roller 303.
In such a sheet supply apparatus, for example, sheets (for a copying machine) having small bending elastic module are separated one by one when a tip end portion of the uppermost sheet is flexed and rides over the holder pawls 302, as is in the conventional separation means of pawl separation type. On the other hand, regarding thick sheets (such as envelopes, post cards) having great bending elastic modulus, the abutment member 305 is greatly flexed by the tip ends of the sheets, with the result that the sheets are successively advanced while sliding on the flexed abutment member. Consequently, the thick sheets are separated one by one. In this way, various kinds of sheets each having different bending elastic modulus can be separated.
Further, as shown in FIG. 30, a thick sheet separating plate 306 may be provided in association with the reference surface. In this case, the thick sheets are separated one by one when the uppermost sheet rides over the separating plate 306 and flexes the abutment member 305.
Further, the Japanese Patent Application Laid-open No. 2-193834 (1990) discloses a technique for separating sheets one by one by using a member similar to the above-mentioned abutment member. In this technique, a sheet stacking plate on which sheets are stacked is urged against a sheet supply roller by springs so that, when the sheet supply roller is rotated, the sheets can be fed out. An abutment member is disposed perpendicular to a sheet supplying direction so that the sheets fed out by the sheet supply roller can be separated one by one when the abutment member is flexed by the sheets. According to this arrangement, various kinds of sheets each having different bending elastic modulus can be separated one by one.
However, in the above-mentioned conventional sheet supply apparatus, there is a danger of occurring the double-feed of sheets due to the poor sheet separation. For example, if a first sheet is adhered to a second sheet by flash and/or an electrostatic force, when these two sheets abut against the abutment member, these two sheets are advanced together while flexing the abutment member, with the result that these sheets cannot be separated from each other.